Studies citing vitamin D deficiency associated with rheumatoid arthritis (RA) have indicated that vitamin D supplementation might reduce the risk of or enhance therapeutic approaches to treat this disease. The discovery of a new secosteroidogenic pathway initiated by P450scc that produces vitamin D3 hydroxyderivatives has opened new options in treatment for RA. This first and main product of the pathway, 20(OH)D3, is non-toxic at concentrations as high as 30?g/kg. We have approached the molecular pathogenesis of RA under the thesis that RA is an autoimmune disorder characterized by T cell dysregulation and that the use of vitamin D analogs can enhance the immunomodulatory effects of T cells. For example, T cells activated by altered peptide ligand (APL) peptide of the immunodominant epitope of type II collagen (CII), ie A9, CII245-270 (A260, B261, N263), upregulate the expression of the vitamin D receptor (VDR) so that both 1,25(OH)2D3 and 20(OH)D3 enhance the suppression of the inflammatory responses by inhibiting Th1 and Th17 responses while increasing the production of Th2 cytokines and IL- 10. We feel that the net result of combining therapies with both analog peptides and vitamin D will be a synergistic downregulation of the severity of arthritis at lower and safer doses than either therapy alone. Yet the mechanism of this suppression remains unknown. Our central hypothesis is that T inhibitory cells induced by the APL upregulate the Vitamin D receptor (VDR) allowing vitamin D3 to act directly on T cells to enhance both the suppression of inflammatory cytokines and the secretion of Th2/regulatory cytokines ultimately leading to suppression of autoimmune arthritis; and that the noncalcemic 20(OH)D3, will be as effective and less toxic than the classical form of vitamin D3 [1,25(OH) 2D3]. To understand the mechanism by which vitamin D enhances this response we propose the following specific aims: Aim 1) To test the hypothesis that redirecting the T cell cytokine profile is a mechanism by which 20(OH)D3 enhances the effectiveness of an APL in attenuating collagen-induced arthritis, Aim 2) To test the hypothesis that modulation of autoimmune arthritis can be enhanced by combining two interventions[ 20(OH)D3 and an APL], and Aim 3) To test the hypothesis that 20(OH)D3 intersects the alternate T cell signaling pathway and affects APL driven T cell cytokine production. Successful completion of these experiments will demonstrate if vitamin D and analog peptides work in synergy to suppress autoimmune arthritis and whether the noncalcemic form of vitamin D, 20(OH)D3 is as effective as 1,25(OH)2D3.